We have developed a series of monoclonal antibodies to human granulocytes, monocytes, platelets, and eosinophils. Characterization of the patterns of reactivity of these antibodies and isotyping of these antibodies has been performed by radioimmunoassays, enzyme-linked immunosorption assays (ELISA), immunofluorescence by flow cytometry and cytotoxicity. Characterization of the antigens identified by these monoclonal antibodies will be performed by SDS-PAGE. Patterns of reactivity with bone marrow cells and human stem cells are also under investigation. These antibodies will also be studied for their effect on phagocytosis, chemotaxis, cytotoxic activity against tumor target cells, and natural killer activity. Platelet antibodies have been demonstrated to inhibit aggregation, adherence, and serotonin release of platelets.